This invention relates to an electronic power supply unit known as the switching-mode power supply (SMPS) such as a DC-to-DC converter having a feedback loop for holding the output voltage constant. More specifically, the invention pertains to such a power supply featuring provisions for overcurrent protection.
A typical conventional flyback SMPS (shown in FIGS. 1 and 2 of the drawings attached hereto) includes a transformer having a primary winding coupled to the pair of DC input terminals via an active switch, and a secondary winding coupled to the pair of DC output terminals via a rectifying diode and smoothing capacitor. The DC input voltage is turned on and off as the active switch is driven by pulses that have been width-modulated by a feedback circuit monitoring the DC output voltage. The feedback circuit includes a switch controller for on/off control of the active switch according to an output voltage detect signal indicative of the output voltage being fed to the load.
Also input to the switch controller is a corrected current detect signal, a corrected or modified version of a current detect signal. The current detect signal itself is a voltage signal obtained by connecting a resistor in series with the active switch. Indicative of the magnitude of the current flowing through the active switch, the current detect signal needs correction preparatory to application to the switch controller in order to hold the maximum output current of the SMPS within acceptable limits in the face of possible fluctuations in the input voltage. Such correction was hitherto implemented with two additional resistors and a capacitor.
A problem has existed in conjunction with these conventional means for correction of the current detect signal because the switch controller together with an associated power supply is usually supplied in the form of an integrated circuit known as a controller IC. Being external to this controller IC, the correcting resistors and capacitor added substantively to the manufacturing cost and size of the SMPS. Additionally, connected between the pair of DC input terminals, one of the correcting resistors caused a power loss that became particularly serious when the input voltage was high.
Japanese Unexamined Patent Publication No. 2005-184882 proposes a different method of making the maximum output current of an SMPS within limits irrespective of changes in the input voltage. It teaches to use both a ramp generator and a rectangular wave generator and to superpose the ramp voltage on the rectangular wave voltage. The ramp voltage and the rectangular wave voltage are both synchronized with the conducting periods of the active switch, and the rectangular wave voltage has an amplitude varying in inverse proportion with the input voltage.
Although capable of lessening variations in the maximum output voltage in response to fluctuations in the input voltage, this second prior art device has its own shortcomings. Besides being a considerable addition to the size of the power supply, the rectangular wave generator is itself configured to detect the input voltage for amplitude-modifying its rectangular wave output accordingly. The result is a power loss in proportion with the square of the input voltage to the serious impairment of the SMPS efficiency.
Another problem heretofore experienced in connection with the SMPS is subharmonic oscillation. Minute variations in the current detect signal due to external disturbances could lead to such changes in the width of the successive control pulses applied to the active switch as might result in oscillation or unstable operation. Subharmonic oscillations were particularly easy to occur when the duty ratio of the width-modulated switch control pulses exceeded fifty percent. Conventionally, this inconvenience has been coped with by superposing a ramp voltage on the current detect signal into what is termed the “slope-corrected current detect signal,” as disclosed in Japanese Unexamined Patents Publication Nos. 2005-184882, supra, and 2004-40856.